Specialty Finishes with Powder Coating
During my training sessions, I get a lot of questions about three-step specialty finishes. Let’s look at the three-step process, and I’ll provide practical information and tips that should be helpful for all coaters. Knowing how to do a three-step finish can improve the quality of your work – even if you never attempt this specific technique.
Although there are other high quality products on the market, I’m going to highlight two separate coatings systems that I have personally used: Tiger Drylac and Prismatic Powders.
Both coatings manufacturers require a clean part that is free from oils, waxes, surface rust, scale, and other contaminants or soils. Most custom powder coating shops satisfy this requirement by blasting the part with sand or some other type of blast media such as garnet or glass. Which process you use depends on how rough your metal is and how much detail you want the coating to highlight.
For example, if you have a steel wheel that has been in the back lot for a year, you are going to have to use an aggressive media to clean all the rust and debris. However, if you are using a laser etched aluminum wheel, you will want as fine a media as possible. For extreme detail, you may just want to use a chemical cleaner or acid-etch so all the surface detail will come through the finish. (For more information about what you need for blasting, click here.)
Chemical pretreatment is frequently used to prepare metal for coating and is sometimes just as necessary as blasting. Because the surface is so slick, aluminum parts and wheels can benefit from a cleaner/sealer that will promote adhesion of the coating. A cleaner/phosphate solution over steel can give additional protection if the coating is ever scratched or chipped in the field. (For more information about pretreatment, read our pretreatment primer.)
Pre-baking the part is usually the next step. This ensures all the water from pretreatment is removed, but drying the part isn’t the only reason pre-baking can help your finishes. If you are coating cast wheels and parts, the casting sometimes traps gasses in the metal. These gasses are only released when exposed to high heat – like the powder curing cycle. When trapped gasses come out during the cure, it creates bubbles and pinholes in the finish. (link to pinholes and outgassing article) By heating the parts prior to coating, the gasses can be released without damaging the finish.
Other metals such at hot-rolled steel, galvanized, or galvaneal can also have gas and oil trapped in them. Steam cleaning might work, but if you still notice oil and bubbles in your finish, start pre-baking your parts. This will help remove the hidden contaminants that can resurface as the part is being baked.
Tiger Drylac Candy System
The Tiger Drylac system recommends a three coat process. The chrome primer, the candy transparent coating, and then the clear topcoat.
Tiger Drylac provides three different primers that can be used, and each will give different end results depending on which color you choose. Of the three, the Kromezone primer seems to be the most reflective. Tiger recommends a full cure cycle on the primer of 10 minutes at 392° F metal temperature.
The next two steps are the tricky part! After the primer has been applied and cured, the candy color is applied at about 2 mils and cured for 5 minutes at 392° F metal temperature. This is not a full cure but rather a pre-gel cure. This helps intercoat adhesion between the candy layer and the final clear layer.
The final coat is the clearcoat and should be sprayed lightly over the candy coat. The cure time for the Tiger Drylac Clear Series 38/00001 is 15 minutes at 392° F metal/substrate temperature.
The last two steps usually require spraying onto a hot part, which can be challenging. Unless you are careful, spraying hot parts can lead to heavier coating thicknesses than you may want. Anytime the part is above 170° F, powder will melt as it starts to contact the metal or the coating that has already been applied to the metal. This can be a good thing, as it will help with deep corners and tough angles that are normally difficult to get powder to electrostatically stick to. However, it can also hurt you by building the final coat too quickly. During this stage remember that the powder output might have to be turned down or your movement may need to speed up as you go through the spray pattern in order to keep the dry mils around 2. You can always just let the part cool down to 150° F or less before spraying to avoid potential issues.
Prismatic Powder Illusion System
The Prismatic Powder system is like the Tiger Drylac system with a few exceptions:
- The Super Chrome primer cure is higher.
- A one-coat system is possible if the metal is shiny to begin with.
- A clear topcoat is not required in some series (PPS series).
- The Illusion system can be used with various gloss topcoats.
When using the Prismatic Powder, the first coat can be a very highly reflective coating they call Super Chrome. This product can be cured at 400° F but seems to work best at about 450° F metal temperature for 12 minutes. They also recommend when you apply the coating to set your kV control to 45. Higher voltage could cause the metallic content in the coating material to develop unusual patterns as it is applied.
The Illusion series from Prismatic Powder has an uncommon gel time which may take you some practice to master. The instructions state the gel time is 2 minutes after the powder flows to a gloss at 400° F. What that means is you set your oven to 400° F and keep an eye on the parts as they cure. As soon as the powder melts to a gloss, start a 2-minute timer. When the timer is done, pull the part out and let it cool to 150° F.
Watch your parts carefully during this process. The metal thickness of your parts affects when the powder starts to uniformly melt. If you cure the gloss too much, you risk de-lamination of the final clear coat. This is a perfect example of why you should always test on some scrap metal or broken parts to get your process debugged before trying it on production parts
After this Illusion Purple basecoat cooled down, I applied a clear. I used Casper Clear, a low-gloss clear topcoat. The application was about 2 mils at a 35 kV setting.
The reason for the lower setting is because the part is somewhat insulated by the previous two coats, so it’s better to treat it like a repaint. The reason you turn down your kV setting for repaints is so you don’t build up too much charge on the part, which can repel the powder in places.
After spraying a light coat I put this wheel back in the oven at 415° F for 25 minutes. The data sheet states 400° F part temperature for 10 minutes. In order to reach that temperature for that time, the wheel had to pre-heat for 15 minutes so it would reach 400° F. Then it remained in the oven for 10 minutes to get the correct cure. I set the oven higher because it would have taken the wheel longer to get to 400° F (30 minutes or longer) if I had set it at 400° F. This because part temperatures climb very, very slowly once their surface temperature is within a few degrees of the air temperature inside the oven.
Here is an example of a header we did with a neon green Illusion system and the same low gloss topcoat.
Many effects can be produced using multi-stage powder coating but the key steps are always:
- Proper metal preparation
- Correct application technique
- Detailed curing plan
- Developing a repeatable process
With any coating process you perform as a professional, check for proper cure and adhesion before releasing a part to your customer. Remember, your reputation can be severely damaged by poor quality control and negative reviews can be tough to counter.
If you found this article helpful, our articles on outgassing/pinholes and pretreatment may help you troubleshoot your finishes. For even more tips and tricks, equipment guides, and much more, check out our Resources page.